DiscoveryHD has a show on now called "Mars Underground" which covers the idea that it's possible to Terraform Mars.

Mars has an atmosphere 1% of that on earth, and as a result of that and it's distance from the sun, it's peak temperature is about 0 degrees celcius.

But based on what we've learned in studying our own global warming through CO2 pollution, we have learned the atmospheric process that would be required to "pollute" Mars with greenhouse gases needed to create an atmosphere on Mars.

The calculations are that it would take about 100 years of modern "polluting" designed to create these gases on Mars.

At which point, the planet would be properly warmed enough to melt the frozen water on the planet and that naitive life on Mars would generate. If there were not naitive life in the frozen water, we could then transport our own trees, planets, etc.

That's where today's science runs out of capability. Because if it were to happen naturally, the process of using that plant life to generate sufficient oxygen for people to live without artificial respiration would take about 10,000 years.

Pretty cool idea that we've learned enough about our own eco-system that we might have the capability today (with sufficient investment) to manually warm a frozen planet through creating a natural atmosphere.

The triology Red, Green and Blue Mars by Kim Stanley Robinson goes over this along with all the politics that might happen.

__________________Even a superstitious man has certain inalienable rights. He has a right to harbor and indulge his imbecilities as long as he pleases. . . He has a right to argue for them as eloquently as he can, in season and out of season. He has a right to teach them to his children. But certainly he has no right to be protected against the free criticism of those who do not hold them. He has no right to demand that they be treated as sacred. He has no right to preach them without challenge." -H.L. Mencken